1. Field of the Invention
The present invention relates to semiconductor wafer handling and processing equipment, and in particular, to a method and apparatus for positioning and orienting an end effector of a wafer handling robot with respect to a wafer to be extracted from a cassette, and for thereafter reading an indicial mark on the wafer.
2. Description of the Related Art
Standardized mechanical interface (SMIF) systems, first proposed by the Hewlett-Pac card Company and disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389, have now become accepted clean room equipment for semiconductor manufacturing. The purpose of the SMIF system is to reduce particle fluxes onto articles, for example, semiconductor wafers. This end is accomplished, in part, by mechanic ay ensuring that during transportation and storage the gaseous medium (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) canopies placed over cassette pots and wafer processing areas of processing stations so that the environments inside the pods and canopies (upon being filled with clean air) become mixture clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from the sealed pods to the processing equipment without contamination of the wafers in the wafer cassette from external environments.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called "bottom opening" pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide "front opening" pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell itself.
In order to transfer wafers between a SMIF pod and a process tool within a wafer fab, a pod typically loaded either manually or automatedly onto a load port on a from of the tool. The process tool includes an access port which, in the absence of a pod, is covered by a port door. Once the pod is positioned on the load port, mechanisms within the port door unlatch the pod door from the pod shell and move the pod door and port door together into the process tool where the doors are then moved away from the wafer transfer path and stowed. The pod shell remains in proximity to the interface port so as to maintain a clean environment including the interior of the process tool and the pod shell around the wafers. A wafer handling robot within the process tool may thereafter access particular wafers supported in wafer slots in the pod or cassette for transfer between the pod and the process tool. Alternatively, a bare cassette (without the pod) may be loaded directly onto the interface load port and transferred into the processing station by the wafer handling robot.
As wafers move through the various processing chambers within a semiconductor wafer fab, it is desirable to be able to track and locate a particular wafer at any given time. Moreover, it is desirable to be able to identify a particular wafer during wafer fabrication to ensure that the wafer is subjected only to processes appropriate for that wafer. This wafer tracking is accomplished by marking each wafer with an optical character recognition (OCR) mark, or similar indicia mark, which mark is read for each wafer prior to locating a wafer within a processing station. The indicial mark is typically a number or letter sequence etch into an upper surface of a wafer near the outer circumference by a laser or other suitable etching means. The indicial mark may alternatively be a bar code or a two dimensional dot matrix at an outer circumference of the wafer.
In order to read the indicial mark on a particular wafer, the indicial mark is conventionally positioned under an image identifying device such as a video camera, which acquires a computer-recognizable image of the indicial mark. The indicial mark must be precisely positioned under the video camera in order for the camera to acquire the image. This requirement is made more difficult by the fact that indicial marks are very small, so as not to take up space on the wafer otherwise sable for circuit devices.
Before an indicial mark may be read, the mark must first be located. When a wafer is seated within a wafer cassette, the orientation of the wafer to the cassette and to a tool for extracting and supporting the wafer is generally unknown. attempts have been made to align the indicial mark of each wafer at a particular rotational orientation within the cassette. However, because wafers move within a cassette upon handling and transfer of the cassette between processing stations, alignment of the indicial marks prior to transportation has not proved feasible. Conventionally, a separate operation has been devoted to orienting a wafer to a known location, locating the indicial mark, and aligning the indicial ark under the camera at or immediately prior to each station where it is desired to identify the particular wafers to be processed in that station.
In order to locate an indicial mark, wafers are conventionally formed with a notch, or flat on the outer edge of a wafer. For each wafer being processed, the indicial mark is located in a fixed, known relation to the notch, and by finding the notch, the precise location of the indicial mark may be determined. Conventionally, in order to locate the notch, the center of the wafer first has to be identified Thereafter, the wafer is rotated on center until a sensor proximate to the rotating wafer edge detects the notch.
FIG. 1 shows a conventional station 20 for performing this operation of wafer centering, notch location, and indicial mark reading. Such stations are conventionally located immediately upstream or as part of each processing station in the wafer fabrication process where the indicial mark is to be read. The station 20 includes a wafer handling robot 22 for accessing and transferring wafers 40 from a cassette 38. The robot 22 includes a shaft 26 mounted for rotation and translation along a z-axis concentric with the shaft axis of rotation. The robot 22 further includes a first arm 28 affixed to an upper end of shaft 26 for rotation with the shaft, and a second arm 30 pivotally attached to the opposite end of the first arm 28. The wafer handling robot further includes an end effector 32 Pivotally attached to the second arm 30. The robot 22 is controlled by a computer 36 such that end effector 32 slides into the wafer cassette 38 underneath one of the wafers 40, rises up to support the wafer 40, and thereafter retracts from the cassette with the wafer 40 supported thereon.
The robot 22 next transfers the wafer to an alignment module 42. The module 42 includes a table 44 capable of rotation and translation in a direction indicated arrow A--A in FIGS. 1 and 2A-2C. The robot 22 deposits the wafer on pins 52FIGS. 2A-2C) around table 44, which pins thereafter retract to rest the w the table 44. Wafer 40 is then rotated on table 44 t o determine the radial of the wafer (i.e., the distance by which the center of the wafer deviates from the axis of rotation of table 44).
In order to determine the radial run out of wafer 44, the module 42 includes a sensor 48 having a plurality of optical transmitters 44a and a plurality of optical receivers 48b. After table 44 rotates wafer 40 360.degree., the computer 36 is able to determine the center of wafer 40 via the sensor 48.
Thereafter, the table rotates to align the axis of maximum radial runout with the direction translation of the table 44 (arrow A--A). Once the wafer 40 is positioned the pins 52 rise up and lift the wafer off of the table 44 (FIG. 2B). Table 44 then translates in a direction along arrow A--A to align the center of wafer 40 with the axis of rotation of table 44. Pins 52 then once again lower to deposit the wafer back onto the table 44, and table 44 translates back to its initial position FIG. 2C). The wafer 40 is then rotated on table 44 to ensure that the center wafer is aligned with the axis of rotation of the table. Once the center of the wafer has been determine d and aligned with that of table 44, wafer is rotated until the sensor 48 senses a notch formed in the wafer as de scribed above. The indicial mark is located in a known orientation with respect to the notch. As such, once the location of the center of the wafer and the location of the notch are known, the indicial mark may thereafter be positioned under and read by a video camera 46 also mounted on the module 42.
After the indicial mark has been read by the camera 46, the pins 52 lift the wafer off of the table 44. The end effector then slides in under the wafer, lifts the wafer off of the pins 52, and returns the wafer to the cassette 38. The entire operation is then repeated on the next subsequent wafer in the cassette.
The above-described conventional process of transferring a wafer to a prealignment module, identifying the center of a wafer, aligning the center of the wafer with the axis of rotation of table 44, and then positioning and reading the indicial mark is a time consuming process. Moreover, this process must be performed a each station where an indicial mark reading is required, and must be perform on each individual wafer at each of these stations. Another disadvantage conventional wafer centering and notch finding systems is that the alignment module conventionally used for determining and re-orienting the center of a wafer takes up critical space within the station 20. Station 20 must be provided with a clean room environment, which is difficult and expensive to maintain. As such it is important that the space within such an environment be used efficiently. Further still, conventional systems as described above determine an align the center of a wafer only after it has been extracted from the wafer cassette. However, initial extraction of a misaligned wafer may result in contact of the wafer with the sides of the cassette, which may result in damage to, or a complete loss of, that wafer.
For certain applications, it may be desired to center a wafer within a cassette, without necessarily having to read an indicial mark. In order to accomplish this at present, a wafer must be extracted from the cassette by a wafer handling robot, placed on a prealignment module 42 described above with respect to FIG. 1, and then returned to the cassette. Again, such a process is time consuming and the module takes up critical space within a clean room environment .